1. Field of the Invention
The present invention relates to a vehicle, which is capable of switching between a front-wheel-drive mode and a rear-wheel-drive mode, as well as to a steering apparatus for use in such a vehicle.
2. Description of the Related Art
There has been proposed in the art an electric power steering apparatus for changing a force that assists the driver of the vehicle in steering the wheel, depending on a state of switching between a four-wheel-drive mode and a two-wheel-drive mode (see Japanese Laid-Open Patent Publication No. 61-207274, hereinafter referred to as “JP61-207274A”, and Japanese Laid-Open Patent Publication No. 63-116982, hereinafter referred to as “JP63-116982A”).
According to JP61-207274A, a ratio of a steering assistive force to a steering force is changed depending on the four-wheel-drive mode or the two-wheel-drive mode (see claims). As disclosed in JP61-207274A, an electrically operated power steering apparatus is premised on a four-wheel-drive vehicle, which is based on a front-engine rear-wheel-drive vehicle (FR vehicle) that drives the rear wheels at all times, and a four-wheel-drive vehicle, which is based on a front-engine front-wheel-drive vehicle (FR vehicle) that drives the front wheels at all times.
According to JP63-116982A, an electrically operated power steering apparatus includes a solenoid-operated valve for controlling a steering force depending on the running state of the vehicle, and a selector for selecting a four-wheel-drive mode or a two-wheel-drive mode, wherein the solenoid-operated valve is controlled based on a signal from the selector (see claims). As disclosed in JP63-116982A, the four-wheel-drive mode or the two-wheel-drive mode is selected by engaging or disengaging a solenoid-operated clutch in order to change paths along which a drive force from an engine 10 is transmitted (see FIG. 2, page 3, lower left column, lines 2 through 4 to page 3, lower left column, lines 15 through 18).
There has also been proposed a four-wheel-drive vehicle, which is capable of independently driving the front wheels and the rear wheels (see U.S. Published Patent Application No. 2012/0015772, hereinafter referred to as “US2012/0015772A1”). According to US2012/0015772A1, a drive unit 6, in which an internal combustion engine 4 and an electric motor 5 are arranged in series, drives the front wheels Wf, whereas electric motors 2A, 2B drive the rear wheels Wr (see FIG. 1, paragraph [0084]).
According to JP61-207274A and JP63-116982A, the front wheels and the rear wheels are driven by a single drive source, with at least the front wheels or the rear wheels being driven at all times. In particular, therefore, according to JP61-207274A and JP63-116982A, nothing is discussed concerning the application of a steering assistive force in a vehicle configuration in which the front wheels and the rear wheels are driven independently of each other. For example, JP61-207274A and JP63-116982A disclose nothing concerning the application of a steering assistive force upon switching between a mode in which only the front wheels are driven and a mode in which only the rear wheels are driven.